be sure to leave enough room for renders too!
If you know you will be rendering anyway you might consider recompressing to a SHEER video codec, the are not RT enabled in FCP but the file size is about one third the file size of regular 10 bit uncompressed.
Please use FCP submit feedback to request that this codec be RT enabled in FCP it would be a fantastic space saver and it is still uncompressed.
http://www.bitjazz.com/sheervideo/
David
Peace and Love :)